<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    #elem-container{
      position: absolute;
      left:     100px;
      top:      200px;
      height:   100px;
      background-color: pink;
    }
</style>
<body>
    <div id="elem-container">dummy</div>
    <div id="output">123</div>
    <script>
        function getTheStyle(){
            let elem = document.getElementById("elem-container");
            let theCSSprop = window.getComputedStyle(elem,null).getPropertyValue("height");
            console.log(theCSSprop);
            document.getElementById("output").innerHTML = theCSSprop;
        }
        getTheStyle();
    </script>
</body>
</html>